With the renewed interest in electric cars we have seen a number of new developments in battery technology, fast charging techniques and wireless power transmission as a convenient method to recharge batteries. Wireless fast charging techniques become even more relevant for pure electric cars as a method to alleviate the limited range provided by current battery technology. In this way batteries could be recharged either while driving from coils embedded in the roads, at traffic lights, in parking lots while shopping or at drive-ins.
Wireless power transfer has a long history starting probably with Tesla. The technology is now used everywhere, from toothbrushes, cell phones, notebooks and is even considered for general use in houses such as lamps, clocks, etc. In most applications wireless power transfer is used for charging batteries, which is used as a temporary energy reservoir between the wireless charging system and the device. With the advent of better battery technologies, such as lithium-ion cells, it becomes feasible to charge batteries much more rapidly than before and to do so with wireless fast chargers. To achieve general acceptance, these wireless fast chargers need to be efficient and robust, which is the focus of some of the applications discussed in this document.
There are many types of wireless power transfer. This disclosure focuses on Resonant Induction Charging (RIC), although much of what is described also applies to other types of wireless charging methods. RIC, as the name implies, uses high-Q tuned coils and capacitors, and power is transmitted from coil to coil through magnetic fields. RIC differs from far-field techniques involving, for example, very high frequency RF fields, which require sophisticated electronics, and near-field techniques, which only work within a fraction of a wavelength when using RIC. With RIC, it is found that significantly more power can be transferred between coils and up to a distance exceeding several coil diameters. Using a magnetic field rather than a radiating electromagnetic field also presents fewer potential health hazards.
A common type of coil used for RIC is a pancake coil with a single spiral winding arrayed in a plane. The circuit diagram in FIG. 1 shows a typical circuit used for RIC, where coils L1 and L2 would be the transmit and receiver coils, respectively, fabricated as pancake coils. As it is the case for transformers, the electrical characteristics of the coils can be described by the coils' resistances, self-inductance, and mutual inductance. The mutual inductance is related to how much of the field generated by one coil traverses the other coil(s), which is largely related to the geometry of how the coils are orientated with respect to each other, including distance and orientation. As the coupling decreases, less of the power is transmitted while the power loss in joule heating remains the same or increases, and hence the efficiency decreases.